This is an application to test a novel hypothesis on the pathogenesis of rheumatoid arthritis. We propose to examine the possible role of p53 mutations in hyperplastic expansion of the synovium as a consequence of inflammation. Reactive oxygen species (ROS) and nitric oxide (NO), present in the inflamed joint, are known to be active inducers of apoptosis, which we propose normally offsets tissue expansion induced by growth factors at the site. Based on studies on precancerous skin lesions, we propose that mutations in p53 can occur as a consequence of long term exposure to the mutagenic action of ROS and NO, and further, that such mutations produce a state of resistance to apoptosis cause by ROS-and/or NO- induced DNA damage of synoviocytes. This resistance to apoptosis results in massive accumulation of cells despite modest proliferation of the tissue, and this expansion can occur in the fact of apoptosis of wild-type cells at the site. With time, the resulting pannus contributes to joint damage. Thus, while we consider rheumatoid arthritis a predominantly immunologic disease, we believe that nonimmunologic events (such as somatic defects in p53-mediated apoptotic response) can make a major contribution to the progression of the disease. We propose to test this model in three ways. First, we plan to examine the role of p53 in apoptosis inducted by ROS and NO in the relevant cell types (macrophages, fibroblast-like synovial cells) and related cells. This will be done in culture, through the use of temperature-sensitive p53 mutants that permit conditional p53 function in a variety of cells. Since p53 is required in many cells for the induction of apoptosis following DNA damaging agents, we expect that it will have such a role here as well. Concurrently, we will examine tissue from rheumatoid joints and controls for the presence of mutation in p53, both by immunohistochemistry and by molecular approaches. These methods have been developed for use in cancer research but have not yet been applied to inflammatory disease. Finally, we will assess the role of p53 in apoptosis and disease progression in mice lacking this gene through targetted disruption, in which we will induce joint inflammation by immunologic means. Together, these studies will form the basis for rigorous pursuit of this model or adjusted models. Although the proposed studies are high risk, they are fully feasible in a three year period, and if successful they will have important consequences for our understanding of rheumatoid arthritis.